In general, transfer sheets are composed of a synthetic resin sheet and have a transferring function for causing images formed on a photosensitive drum to be transferred thereonto. In some cases, the transfer sheets also have a transporting function for causing the images carried on the photosensitive drum to be transported to copying papers and a separating function for causing the copying papers on which the images have already been transferred, to be separated therefrom. The transfer sheets are generally used in the form of a drum or a belt, and always under go a mechanical load during the use. For this reason, it is required that the transfer sheets exhibit a high strength against repeated loading, i.e., a high fatigue strength in addition to a so-called mechanical strength and elongation.
It has been strictly required that the afore-mentioned mechanical durability of the transfer sheets is so high as to withstand several ten thousands of copying operations. The mechanical durability of the transfer sheets varies depending upon structure or configuration of the copying machines or printers in which the transfer sheets are used, and therefore is evaluated by the below-mentioned testing methods. The transfer sheets are required, if possible, to exhibit a mechanical durability capable of withstanding not less than a hundred thousand copying operations when evaluated by such testing methods. However, it has been considered by a person skilled in the art that the afore-mentioned high mechanical durability of the transfer sheets is extremely difficult to realize by using an existing moldable synthetic resin sheet.
Further, since the transfer sheets are used as an electrical part to which a high voltage is applied, it is inevitably required that they have flame-retardant properties. The flame-retardant properties required for the transfer sheets are as high as not less than VTM-2 according to Underwriters Laboratories (UL) Standard No. 94 (Underwriters Laboratories Inc.). However, the increase in flame-retardant properties of a resin material causes the deterioration in its mechanical strength, while the increase in the mechanical strength causes the deterioration in the moldability and the flame-retardant properties. Therefore, it is extremely difficult to satisfy these properties simultaneously.
The present invention has been accomplished in view of the afore-mentioned problems. It is an object of the present invention to provide a transfer sheet which satisfies a high mechanical durability, high flame-retardant properties and a high moldability, simultaneously.